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Am J Physiol Gastrointest Liver Physiol 284: G37-G45, 2003. First published September 18, 2002; doi:10.1152/ajpgi.00209.2002
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Vol. 284, Issue 1, G37-G45, January 2003

cAMP-mediated regulation of murine intestinal/pancreatic Na+/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter subtype pNBC1

O. Bachmann1, H. Rossmann1, U. V. Berger2, W. H. Colledge3, R. Ratcliff3, M. J. Evans4, M. Gregor1, and U. Seidler1

1 Department of Internal Medicine, University of Tübingen, D-72076 Tübingen, Germany; 2 In Situ Hybridization Core Facility, Beth Israel Deaconess Medical Center, Boston, MA 02115; 3 Department of Physiology, University of Cambridge, Cambridge CB2 3EG; and 4 Cardiff School of Biosciences, Cardiff CF10 3U5, United Kingdom

Basolateral Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport is essential for intestinal anion secretion, and indirect evidence suggests that it may be stimulated by a rise of intracellular cAMP. We therefore investigated the expression, activity, and regulation by cAMP of the Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter isoforms NBC1 and NBCn1 in isolated murine colonic crypts. Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> transport rates were measured fluorometrically in BCECF-loaded crypts, and mRNA expression levels and localization were determined by semiquantitative PCR and in situ hybridization. Acid-activated Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport rates were 5.07 ± 0.7 mM/min and increased by 62% after forskolin stimulation. NBC1 mRNA was more abundant in colonic crypts than in surface cells, and crypts expressed far more NBC1 than NBCn1. To investigate whether the cAMP-induced Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport activation was secondary to secretion-associated changes in HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> or cell volume, we measured potential forskolin-induced changes in intracellular pH and assessed Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> transport activity in CFTR -/- crypts (in which no forskolin-induced cell shrinkage occurs). We found 30% reduced Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> transport rates in CFTR -/- compared with CFTR +/+ crypts but similar Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport activation by forskolin. These studies establish the existence of an intracellular HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> concentration- and cell volume-independent activation of colonic NBC by an increase in intracellular cAMP.

bicarbonate secretion; colon; ion transport; chloride secretion; 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein


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